Looked up the specs, and the standard supported speed is PC2700, which is 333MHz RAM. 400 will work, but it will be slowed down to 333 to match the standard. Make sure that the RAM you're getting is DDR.

You just pop it in the RAM slot, and boot up the computer. You can check the BIOS to make sure that the right amount of memory has been installed. And you can also right click My Computer, and properties to make sure it's being recognized.

Usually though, if the RAM stick is faulty, the computer won't boot up.